OK. This is because the exposure system makes an assumption about 18% grey and nothing to do with TTL-BL specifically right? The snow is the problem. I think that is the main point for me.
However on the advanced problem... my understanding is something like this: the camera can measure the amount of light reflected from a scene. But it doesn't know anything about the surfaces doing the reflecting. So it has to make an assumption that all scenes are made up of 18% grey.
In the case of snow it gets a lot of light back but still makes the assumption that the scene is 18% grey. Therefore it thinks the scene is brighter than it is. (If 18% grey was reflecting that much light it would be really bright). So it underexposes.
The main thing then was that it is the snow that means I may have to add EV by one or two stops not something to do with TTL-BL.
I am interested in your comments about Program mode. I've never used it assuming it is for amateurs. Perhaps I should try.